Polymer electrolyte fuel cells (PEFCs) have great potential as an environmentally friendly energy source. Fuel cells have been used in the space program since the 1960's, but recently with the focus on “green” resources, fuel cells have come to the forefront of commercialization. Specifically, fuel cells are being explored for use in automobiles, electronics, and stationary power applications.
Perhaps the most critical component of the fuel cell is the proton exchange membrane (PEM). For the last 30 years, the industry standard for the PEM component of the fuel cell has been Nafion® (polyperfluoro sulfonic acid) by DuPont.
Nafion® membranes display sufficient proton conductivity (˜0.1 S/cm), good chemical resistance, and mechanical strength. Some of the membrane's disadvantages include high cost, reduced conductivity at high temperatures (>80° C.), and high methanol permeability in direct methanol fuel cells.
Increasing the operation temperature of fuel cells is important for several reasons. Firstly, higher operating temperatures in the fuel cell decreases the carbon monoxide poisoning of the electrocatalyst. Carbon monoxide in concentrations of a few parts per million can adversely affect performance at temperatures around 80° C. Secondly, higher temperatures increase reaction kinetics of hydrogen oxidation on the anode and oxygen reduction on the cathode. However, as the temperature is increased, it becomes more difficult to keep the membrane hydrated. Dehydrated membranes lose ionic conductivity and result in poor contact between fuel cell components due to shrinkage of the membrane. The challenge is to produce membranes not limited by the temperature range of liquid water.
Because of the renewed interest in fuel cells and the challenge of high temperature operation, new membrane materials have been explored as potential replacements for Nafion®. Previous work has focused on sulfonated polystyrene, styrene-butadiene block copolymers, or poly(arylene ether)s such as PEEK. Typically, these polymers were all made by a post-sulfonation polymer modification reaction where the sulfonic acid groups are attached to the already formed polymer backbone.
Sulfonated poly (arylene ether sulfone)s made from post-polymerization sulfonation reactions have been of interest since the pioneering work of Noshay and Robeson, who were able to develop a mild sulfonation procedure for the commercially available bisphenol-A based poly(ether sulfone). This approach found considerable interest in the area of desalinization membranes for reverse osmosis and related water purification areas. In the post-polymerization sulfonation reaction, the sulfonic acid group is restricted to certain locations on the polymer chain. In this example of the bisphenol A based systems illustrated as Structure 1, the sulfonic acid group is almost always restricted to the activated position ortho to the aromatic ether bond. Additionally for this system, only one sulfonic acid group per repeat unit is typically achieved.
Structure 1. Bisphenol A Based System where x is —C(CH3)2—.
